American Foulbrood - Biology
and Diagnosis

American foulbrood (AFB) is the most serious brood disease of honey
bees. AFB is caused by a spore-forming bacteria, Paenibacillus
larvae; that is specific to honey bees. This disease is highly
contagious, will weaken and in most cases kill a honey bee colony.
AFB will also contaminate beekeeping equipment which must then be
destroyed to prevent the spread of AFB to additional colonies. There
is no cure for AFB. Beekeepers can only take steps to prevent an
infection from establishing itself in a beekeeping operation.

Note: In the past AFB was known by the species name Bacillus
larvae.

Prevalence

AFB may be found at any time of the year in honey bee colonies
or in used beekeeping equipment infected with bacterial spores.
Used equipment infected with AFB can remain contaminated for decades.

There are strains of AFB present outside Ontario that may be resistant
to currently registered drug treatments for AFB such as oxytetracline.
Although AFB is found throughout Ontario, there have been no instances
of AFB resistant to oxytetracycline in Ontario. To maintain this
status, beekeepers must adhere to requirements when importing bee
stock from other provinces or countries. Beekeepers must also take
care not to misuse antibiotics. See Treatment
Recommendations.

Impact

AFB has the potential to contaminate beekeeping equipment so it
can no longer be safely used. In most situations AFB is highly virulent
and once contracted will kill a colony. An AFB infection can spread
to other honey bee colonies up to an 8 km radius (typically 3.2
km). AFB can spread anywhere infected colonies are located and whenever
contaminated equipment is exchanged, no matter what the distance.
One beekeeper's AFB infection can quickly become another beekeeper's
problem. This risk makes AFB an economic threat to commercial beekeepers.

Life cycle and biology

AFB has two major life stages; the vegetative stage and the spore
forming stage. AFB will only impact developing honey bee larvae.
Adult honey bees are not affected. (See figures 1 to 3)

1. Spore stage

AFB spores are picked up by worker bees from infected honey bee
colonies or equipment contaminated with large numbers of spores.
These spores are highly resistant to heat and other environmental
factors and can remain viable in contaminated used honey bee equipment
for up to 70 years.

2. Vegetative stage

Spores contaminate the mouthparts of worker bees and are fed to
developing honey bee larvae via brood food (less than 2 days old).
The spores undergo a transformation into the vegetative stage of
the bacterium, rapidly multiplying in the tissues of the larvae,
generating billions of new spores.

3. Spore stage

The honey bee brood dies after the brood cell is capped then decomposes
into a gooey mass, referred to as "AFB scale". This scale
settles to the bottom of the cell. Eventually the scale dries, becomes
black in color, hardens and adheres firmly to the wall of the wax
cell. Each scale may contain up to 2.5 billion spores. As only 35
spores will infect a day old bee larva and only one spore can infect
an hour old larva the capacity for infection is enormous.

4. Spore stage

As the worker bees remove diseased scale from the wax comb they
will contaminate their mouthparts and infect other developing brood.
Many honey bee brood then become infected. The strength and health
of the honey bee colony begins to break down as the colony's population
dwindles.

5.Spore stage

As the colony becomes weakened and further contaminated by AFB
it may be targeted by nearby honey bee colonies whose bees rob the
honey stores of the weakened colony. This honey is brought back
to healthy honey bee colonies along with large numbers of infective
AFB spores. Spores infect honey bee brood in the new colonies.

Figure 1. Ropey test of AFB infected larvae

Figure 2. AFB infected larva settling to the bottom
of the cell

Figure 3. Old dried AFB infected larvae or AFB scale

Spread of AFB Through Honey Bee Activity

Robbing

Honey bees from one colony may collect the honey stores of another
colony. When this occurs, AFB spores can be transferred with the
honey. This can result in the spread of AFB within a beeyard or
between different beeyards (up to 8 km). This is particularly a
concern during a nectar dearth when combined with weakened colonies
and exposed contaminated wax comb. Infected beekeeping equipment
that is stored in locations accessible to honey bees is another
source of infection.

Drifting

Honey bees from adjacent colonies mistakenly enter another colony
after returning from foraging. This can spread AFB within a beeyard.

Spread of AFB Through Beekeeper Activity

Feeding contaminated honey or pollen

Honey and pollen obtained from outside sources may contain AFB
spores. Honey from outside sources should not be fed to honey bee
colonies. If purchasing pollen for the purpose of feeding to honey
bees, ensure that the pollen has been irradiated.

Exchanging equipment between colonies

Moving frames of brood between different colonies in a beeyard
to equalize colonies or to boost weak colonies is a common beekeeper
practice among beekeepers. AFB may be spread from an infected colony
to an uninfected colony when this is done. Beekeepers must be mindful
of this practices's risk and should be vigilant for any symptoms
of disease.

Purchasing infected honey bee colonies or equipment

Beekeepers regularly sell and purchase honey bees or used equipment
from other beekeepers. This may also introduce AFB to a clean operation.
The selling or purchasing honey bees and of used beekeeping equipment
between beekeepers falls under the legislation of the Ontario Bees
Act. Under the Act a beekeeper selling any honey bees or used beekeeping
equipment must hold a valid permit demonstrating that the material
for sale or a proportion of the beekeeping outfit received an inspection
by a provincial apiary inspector and has been demonstrated to be
in good health.

Packages

Although not as likely to spread AFB, honey bee packages are a
potential source of infection. Packages require federal and provincial
import permits.

Swarms

Swarms may be a source of AFB if the swarm originated from an infected
colony. Swarms always have a chance of transferring pests and diseases
since there is no way to be certain of the swarm's origin. However,
since a swarm has no brood there is an opportunity to take management
steps to minimize the risk.

Detection-Diagnosis

Physical symptoms of AFB are found in the brood nest of the honey
bee colony and used brood comb. It is important for beekeepers to
familiarize themselves with healthy brood conditions and types of
brood disease. There are a variety of symptoms that indicate an
AFB infection. Not all symptoms are always present in each AFB infection,
particularly with the different stages of an infection, and not
all are specific to AFB. However, some symptoms are very specific
to the disease.

Spotty brood pattern

(not always specific to AFB)

A healthy, productive colony will produce a solid brood pattern,
with few empty cells. Unhealthy or diseased colonies may have a
brood pattern with many missed cells. This feature is common to
many disorders in a hive including other brood diseases or may simply
indicate the condition of a poor queen (figure 4).

Figure 4. Spotty brood pattern with noticeable AFB
scale.

Figure 5. Sunken perforated cell cuppings.

Perforated brood cappings of the brood cell

(not always specific to AFB)

It is more likely that the perforated capping of a brood cell is
a brood disease if the perforations are irregular and located at
the margin of the capping. This feature is seen in other brood diseases
such as chalkbrood and brood damage caused by high levels of varroa
mites (figure 5).

Wax cappings with a dark, greasy appearance

(not always present in early stages of AFB).

Wax cappings that are sunken in

(not always present)

Wax cappings will be slightly concave rather than slightly convex.

Discoloured larvae

(not always specific to AFB)

Any time a larvae dies in the cell, it will darken and become discoloured.
Healthy larvae are always pearl white. Infected AFB brood will turn
from white, to beige, to light coffee brown, to dark coffee brown,
then to black as it begins to dry out (figure 6).

Dead larvae exhibiting a gooey, mucus-like consistency that settles
to the bottom of the brood cell

(very specific to AFB, usually present other than very early or
late stages of larvae decomposition)

This is a condition specific to AFB. Place a match stick or thin
twig into the cell, rotate the stick 360° and slowly pull the
stick out of the cell. If the material in the cell stretches out
to a length of 2.5 cm or more, it is infected by AFB. No other brood
disease displays this symptom. American foulbrood does not always
display this symptom. If it is early in the infection, the dead
larvae will not be gooey enough. In later stages the dead larvae
will harden and dry. If this symptom is encountered it is most definitely
AFB (figure 7).

Black hardened scale that adheres to the wall of the wax cell

(This is specific to AFB.) After the diseased larvae settles to
the bottom of the cell it will begin to dry and harden. This is
an important symptom found in used brood comb. Look for black, flat,
charcoal-like pieces stuck to the bottom of the cell. This dried
scale is hard, brittle and cannot be removed from the wax cell without
damaging it (figure 8). The best way to do this is to hold each
frame with your thumb and index finger supporting the ear of the
frame and slowly rotate the frame back and forth so the bottom of
the frame rotates along a 30° axis (figure 9). Make sure that
you have good lighting as you do this, and if possible, have the
sun shine at your back.

Figure 6. Infected larvae settled to the bottom of
the brood cell.

Figure 7. Ropey test. Larva has a muscus-like consistency and will
draw out of the cell.

Figure 8. Rotted, drying AFB scale and greasy, dark wax comb.

Foul smell

(specific / not specific to AFB)

Although other diseases and disorders of honey bees may be associated
with a foul odour, the odour of AFB is quite distinct. It has an
odour described as "fishy" or "rotten". It is
a smell that once encountered is not easily forgotten.

Conclusion

Beekeepers should examine their colonies on a regular basis to
ensure that they are healthy and disease free. While inspecting
the health of brood, gently shake most of the bees off of the frame
over the open hive so that the appearance of the brood can be observed.

Site Help

Notices

Accessing this message means you do not have a JavaScript enabled browser. If you
cannot enable JavaScript in your browser and would like to know the last modified
date for this page, please contact the webmaster at -internet.webmaster.omafra@ontario.ca